Item parameters: a matrix with one row per
  item, and three columns: [,1] item discrimination
  $a$, [,2] item difficulty $b$, and [,3] asymptote
  $c$.

The values of the latent variable ($\theta$
  in the equation above), at which the IIF will be
  evaluated. If not given, 99 values spaced evenly between
  -4 and +4 will be used, handy for plotting.

The item information function (IIF) for the 3PL model can
  be computed as $$I(\theta) =
  a^2\frac{Q(\theta)}{P(\theta)}\left[\frac{P(\theta)-c}{1-c}\right]^2,$$
  where $\theta$ is the value of the latent variable
  for a person, $a$ is the discrimination parameter for
  the item, $P$ is the IRF for the person and item, and
  $Q=1-P$. For the 1PL and 2PL models, the expression
  reduces to $a^2PQ$.

models

Provides a simple common interface to the estimation of
        item parameters in IRT models for binary responses with three
        different programs (ICL, BILOG-MG, and ltm, and a variety of
        functions useful with IRT models.

iif: Item information function

Description

Usage

Arguments

Value

Details

See Also

Examples